how to mark all of your gmail emails "READ"

1. in the search box, type in:unread
2. below the search box, their is a button with an unchecked smaller box.  click on the small triangle there and select "all"
3. Then a small yellow bar appears asking whether you can to select all conversations, click on it
4. click "more" -> mark as read

More details :  http://www.zdnet.com/article/how-to-mark-all-unread-emails-as-read-in-gmail-and-more/